Typically, water resistant cables are formed by jacketing the cable with a water impervious layer comprising a laminated, plastic coated layer, then a "hold" tape layer, then a metal shield layer. The layers of insulation are generally formed of cross-linked polyethylene or the like.
Numerous materials for the "hold" tape have been proposed, with acceptable success; however, the preferred "hold" tapes are quite expensive, and in combination with special laminated plastic/metal foil tapes are a significant cost factor (up to about 25 percent) in the manufacture of the cable.
U.S. Pat. Nos. 3,209,064; 3,315,025; 3,792,192; 3,865,971; 4,008,367; 4,079,190; 4,275,262; 4,383,132; 4,398,058; and 4,703,134 represent some of the known cable designs. U.S. Pat. No. 4,398,058 discloses a moistureproof electrical cable having a corrugated composite tube made of inner copper or aluminum and outer steel portions. This reference teaches that the composite tube is welded to effect a waterproof seal. Such a cable construction is very expensive and may not provide the necessary moisture barrier. Such construction results in a relatively inflexible cable and is subject to possible splitting of the composite tube when the cable is formed into a bend of a small radius or is subjected to thermal expansion. A discussion of alternative cable designs and their limitations is included in U.S. Pat. No. 4,398,058. U.S. Pat. No. 3,209,064 teaches the use of a copper or aluminum tape as an electromagnetic screening material, but does not teach or suggest that the tape be sealed or used as a moisture barrier. U.S. Pat. No. 3,833,132 teaches the use of a copper strip as a conductive coating, but not as a moisture barrier.
U.S. Pat. No. 4,703,134 is believed to represent more recent improvements in moistureproof high power cable designs. In the design illustrated in that patent, a special "hold" tape layer of costly electrically conductive fabric encloses and protects a water impervious layer formed of an elongated metallic material from 20 to 200 micrometers thick. The metal layer is covered on the outer sides thereof with an at least partially conductive plastic film of 20 to 200 micrometers thickness and a volume resistivity of 1 megohm-centimeter or less. A lead or lead alloy metal tape is stated as preferable. A lubricant is preferably applied to the expensive "hold" tape, which is overwrapped with a series of shield wires to form an electrostatic shield. The special "hold" tape provides the important function of substantially greater expansion in a first direction than in a second direction normal to the first. Since high voltage power cables expand and contract diametrically during heat cycling, the special "hold" tape enables such expansion and contraction without rupturing because the special "hold" tape is placed between the laminated metal foil and the shielding wires. The "hold" tape is capable of stretching circumferentially without stretching longitudinally. It is designed to flex between the laminated metal water barrier and the layer of shield wires, thus protecting the laminated metal water barrier from mechanical injury from movement of the shield wires. The special "hold" tape also provides a needed degree of corona-reducing electrical conductivity. The series of electrostatic shield wires is encapsulated within a jacket of polyvinyl chloride 66 kilovolt cross-linked insulation for outer protection.
The cable design of U.S. Pat. No. 4,703,134 is believed to be illustrated in FIG. 6, wherein the cable construction 60 includes a central core 62, a first insulating layer 64; a special, double-sided plastic laminated metal foil barrier 66 having a central metal foil layer 74 and exterior surface coatings 72, 76; and a layer of special "hold" tape 70. An outer layer, not shown, may also be included. Jacket 68 is of polyvinyl chloride, formed as an outermost layer to provide 66 kilovolt insulation.
It is believed that by fabricating the cable as shown in FIG. 6 with the heat-containing metal foil and the special "hold" tape relatively close to the conductor core, certain problems arise in effectively dissipating the heat known to be generated by passage of current through the conductive core. One result of this ineffective heat dissipation is believed to be expansion splits of the cable jackets, and resultant undesirable corona effects leading to premature cable failure.
It is an object of the present invention to provide an improved water resistant cable which avoids the use of expensive component materials.
Another object of the present invention is to enable manufacture of the water resistant cable without expensive, specifically designed equipment or materials.
Another object of the present invention is to provide a water resistant cable construction which permits normal minimum bending radius limitations in ordinary use.
A feature of the present cable invention is that it is designed to meet or exceed the product characteristics of the known cable designs which use special, expensive materials.
Another feature of the present cable invention is that it may easily be manufactured on conventional equipment.
Advantageously, a metal foil moisture barrier sandwiched between two plastic layers reduces foil susceptibility to mechanical damage, resulting in improved moisture protection. By removing the metal foil moisture barrier radially to a location outside of the shielding wires, more effective dissipation of heat is obtained, reducing undesirable corona effects and/or splitting.
An advantage of the present invention is significant material cost savings, consistent with a high quality water resistant cable product.
Another advantage of the present invention is that the novel water resistant cable can be readily manufactured on a conventional cable extrusion line, requiring the addition of only a longitudinal or spiral metal foil tape folder, which may be simply bypassed and not used when manufacturing other cable configurations on the same equipment line.